1. Field of the Invention
The present invention concerns generally a pattern forming method and, more in particular, it relates to a pattern forming method improved such that a satisfactory pattern shape of high resolution power and high sensitivity can be obtained.
2. Description of the Background Art
At present, large scaled integrated circuits (LSI) typically represented by 1M and 4M dynamic random access memories (DRAM) have been prepared by selectively exposing positive photoresist comprising a novolak resin and naphthoquinone diazide to g-ray light from a mercury lamp (436 nm wavelength) and then forming a pattern. The minimum pattern size is from 1 .mu.m to 0.8 .mu.m. However, since it is expected that the degree of integration for LSI will be increased more and more, for example, as seen in 16 MDRAM, a method of forming a pattern by the order of one-half micron size will be required. In view of the above, a pattern forming method using, as an optical source, a KrF excimer laser which is a shorter wavelength optical source has been studied at present.
As photoresist material used for Deep UV light typically represented by the KrF excimer laser, there has been proposed positive photoresist of novolak-naphthoquinone diazide type such as PR 1024 (products manufactured by McDarmid Co.), polymethyl methacrylate (PMMA), polyglycidyl methacrylate (PGMA), polychloromethylated styrene (CMS), etc. PMMA and PGMA show low sensitivity and poor dry etching resistance. CMS has good dry etching resistance but shows low sensitivity. PR 1024 has good dry etching resistance and higher sensitivity as compared with that of the above-mentioned photoresist but its sensitivity is low as compared with by g-ray exposure.
Explanation will then be made for a conventional pattern forming method.
FIGS. 6A-6C are cross sectional views for illustrating a conventional pattern forming method using novolak-naphthoquinone diazide type positive resist material (for example, PR 1024).
As shown in FIG. 6A, PR 1024 is coated on a substrate 2 to be fabricated and then pre-baked to obtain a resist film 1 of 1.0 .mu.m film thickness.
Then as shown in FIG. 6B, KrF excimer laser beam 4 is selectively irradiated to the resist film 1 using a mask 5. Thus, the resist film is partitioned into an exposed area 1a and an unexpected area 1b.
Then, as shown in FIG. 6C, development is carried out by using an aqueous 2.38 wt % solution of tetramethyl ammonium hydroxide to obtain a resist pattern 9 in which an exposed area 1a is removed. In the conventional resist pattern forming method having thus been constituted, there are the following problems.
Since the novolak-naphthoquinone diazide type positive resist such as PR1024 shows great absorption to Deep UV light, light absorption is remarkable at the surface of the resist film and the light can not reach as far as the lower portion of the resist film 1. As a result, a resist pattern 9 has an upwardly pointed trigonal cross sectional shape as shown in FIG. 6C, failing to obtain a fine pattern at good accuracy.
Further, in the conventional pattern forming method using not Deep UV light at a wavelength of 300-500 nm, if the substrate 2 to be fabricated has a step 2a as shown in FIG. 7, light 20 is scattered at the step 2a failing to obtain a satisfactory pattern shape (this is referred to as a notching phenomenon). If a light reflective film such as an Al film is formed on the fabricated substrate 2, there is also a problem that no satisfactory pattern shape can be obtained due to the effect of the reflection light.
FIGS. 8A-8D show another example of conventional resist pattern forming method described in Japanese Patent Laid-Open No. 63-253356.
As shown in FIG. 8A, a resist film of novolak-naphthoquinone diazide type is formed on a substrate 2 to be fabricated. Then, light at 300-500 nm wavelength emitted from a high pressure mercury lamp is selectively irradiated on the resist film 1 by using a mask 5. By the exposure of the light, crosslinking reaction of the resin occurs in the exposed area 1a.
Then as shown in FIG. 8B, by exposure of light of an identical wavelength to the entire surface of the resist film 1, photosensitive agent is completely decomposed.
Subsequently, as shown in FIG. 8C, vapors of trimethyl silyldimethylamine are acted on the entire surface of the substrate 2. This causes selective silylation to a portion excepting for the exposed area 1a, that is, to the upper portion of an unexposed area 1b to convert the portion into a silylated layer 8. Then, as shown in FIG. 8D, when development is carried out by reactive ion etching (RIE) using O.sub.2 gas, the silylated layer 5 remains in the form of a SiO.sub.2 layer 13, while the exposed area 1a is removed, to thereby form a resist pattern 9 on the substrate 2.
However, in this conventional example having thus been constituted, since light at 300-500 nm length has been used, no sufficient crosslinking is conducted for the resin as shown in FIG. 8A. As a result, selectivity of the silylation (ratio between the silylating reaction in the upper portion of the exposed area 1a and the silylating reaction in the upper layer portion of the unexposed area 1b) is low as shown in FIG. 8C. This leads to a problem that the boundary between the exposed area and the unexposed area is not distinctive, failing to obtain a fine pattern at high accuracy as shown in FIG. 8D. Further, this method also involves a problem of the notching effect as described above referring to FIG. 7.